Session: 13-01-01: Design and Fabrication, Analysis, Processes, and Technology for Micro and Nano Devices and Systems

Paper Number: 92967

92967 - Effect of SF6 and C4F8 Flow Rate on Etched Surface Profile and Grass Formation in Deep Reactive Ion Etching Process 

Etching bulk silicon is a critical process in fabricating numerous MEMS devices. Deep reactive ion etching (DRIE) is one of the most important methods of etching silicon and involves numerous interdependent parameters that affect the resulting etching quality and device functionality. Some of the issues in the etching process include bottom grass formation, notching effects, and anisotropic etching. This paper provides a case study based on the Alcatel DRIE system housed at the University of New Mexico (UNM) Manufacturing Training and Technology Center (MTTC). We investigated varying different etch parameters and the effect had on etching rate, grass formation, and etch profile for both 100 mm and 150 mm silicon wafers. The metrology mask used had structures ranging from 5 µm to 20 µm. The influence of SF₆ and C₄F₈ flow rates, as well as etching and passivation cycle duration, on the etched sidewall profile and grass growth is discussed in this paper. This process involves creation of series of experiments for improving process parameters. Optimization entails knowing how each parameter works and how it affects the final outcomes, whereas characterization comprises of examining the results acquired in terms of etch rates, surface profile, and smoothness for silicon plasma etching leveraging surface roughness, scanning electron microscope and other metrology tools. This includes resolving any difficulties that develop during testing and characterization. The combination of a 300 sccm SF₆ flow rate and a 100 sccm C₄F₈ flow rate produces a flat bottom surface with no grass formation. The creation of grass begins in the corners of the structures as the SF₆ flow rate increases above 300 sccm while the C₄F₈ flow rate remains constant (100 sccm) and cycle ratio of 5:1. The flow rate of C₄F₈ was varied from 50 to 200 sccm (with SF₆ at 300 sccm), with lower flow rates affecting the profile of the etched sidewalls and higher flow rates causing grass development. The etch/passivation cycle ratio, which is believed to be critical in the fabrication of high aspect ratio microstructures, has also been discovered to be significant in preventing grass growth. The ultimate etch rates per cycle are determined by polymer deposition rate, polymer etch rate, and silicon etch rate, which are all heavily influenced by the aforementioned factors such as gas flows, pressure, substrate temperature, and cycle ratio. The clamping or loading of the 4-inch wafer had cooling issues that affected the surface profile of the structures and demonstrated that insufficient cooling of the substrate during etching led to the production of isotropic etching of structures and resolved using lubricant media for proper heat dissipation. The process recipes were optimized to achieve flat smooth bottom surface and straight vertical sidewalls with no defects.

Presenting Author: Nathan Jackson University of New Mexico

Presenting Author Biography: Nathan Jackson is an Assistant Professor in the Mechanical Engineering Department at UNM. He received his Ph.D in Biomedical Engineering from Arizona State University. Prior to UNM, he worked at a microelectronics research institute (Tyndall National Institute) located in Ireland as a Senior Researcher and head of the PiezoMEMS team. His research interests are in the areas of MEMS, BioMEMS, piezoelectrics, functional thin film materials, acoustic resonators, and flexible/stretchable circuits. He has developed a wide range of MEMS devices for vibration energy harvesters, particle sensors, atomizers, acoustic resonators, robotics, tactile sensors, microneedles and RF MEMS. He is a technical committee member for IEEE MEMS, SPIE Microtechnologies, E-MRS, IEEE NANO, and IMECE conferences. He is a Senior Member of IEEE and has published >90 peer reviewed journal publications focused on MEMS and functional materials. He has 10 patents licensed to various companies.

Authors:

Pallavi Sharma University of New Mexico
Matthias Pleil University of New Mexico
Nathan Jackson University of New Mexico